The thin film transistor-liquid crystal display (TFT-LCD) is the current main stream flat panel display, its basic structure comprises a liquid crystal screen formed by box aligning of two substrates. An electric field is applied to the liquid crystal between two substrates through the pixel electrode and the common electrode on the substrate, so as to control rotation of the liquid crystal to form the desired image, and enable the voltage to be kept until update of the image next time through the storage capacitor.
In the production process of the liquid crystal screen, firstly, it is required to form various thin film pattern with a fixed size on an entire piece of substrate through patterning process, including pattern of the pixel electrode, pattern of the source drain, pattern of the insulating layer etc., thereby forming a display substrate of a large size, then the display substrate is cut to obtain sub display substrates of various sizes, thereby forming a liquid crystal screen of the desired size. Wherein the larger the size of the display substrate is, the more the number of the sub display substrates obtained from cutting is, the higher the utilization rate and the benefit are, and the lower the manufacture cost of the liquid crystal screen is. However, the existing thin film pattern obtained through deposition technique (PECVD) have edge effect on the display substrate, i.e., the edge of the insulating layer (SiNx film) in the display substrate is thin and the middle thereof is thick, while the edge of the semiconductor layer (non-doped a-Si film) is thick and the middle thereof is thin, such that sub display substrates located at different positions of the display substrate have electrical difference, particularly for sub display substrates located at the edge of the display substrate, the off-state current and the threshold voltage of the thin film transistor thereon are increased, and the on-state current thereof is decreased, thereby causing electrical badness of the sub display substrates located at the edge of the display panel.